What does the medical abbreviation 1 po tid mean?

The term "PO TID" on a doctor's prescription or order would mean: "by mouth, three times daily".

PO means by mouth. This medical abbreviation, which is also written as: p.o.

or po (lower case), comes from the Latin phrase "per os".

The term TID means three times daily, or three times per daily. This medical abbreviation, which is also written as: t.i.d. or tid (lower case), comes from the later phrase "ter in die".